1. Field of the Invention
The present invention relates to an apparatus for supplying air and at least one additional gas to a respirating subject during inspiration phases, of the type having a container which is open to surrounding atmosphere at one end for receiving air, a tubing system connected to the other end of the container and connectible to the subject, a source of an additional gas and a point of entry for the additional gas to mix with the air to be supplied to the subject.
2. Description of the Prior Art
Advanced ventilators (respirators), capable of providing a number of different respiration modes, are usually complex to use and also expensive. Normally, they also require reliable power sources and are quite heavy. Together this makes it difficult to use this kind of ventilator in home care equipment or as resuscitation aid for ambulances, air craft and similar locations. Such equipment preferably should be easy to use and at the same time be reliable.
A reliable light weight and low cost ventilator is therefore desirable. Such an apparatus should be able to provide atmospheric air and at least one additional gas, such as oxygen.
An apparatus of this kind is described in German OS 1 233 537. The apparatus disclosed therein has a container having an open end toward ambient air and a connection to a tubing system leading to a patient at the other end. Between the container and the patient an additional gas such as oxygen can be provided.
The additional gas is supplied in a continuous flow. During spontaneous inhalation the patient will draw gas from the container, which then is mixed with the additional gas. During expiration the additional gas will flow toward the container (which is open to ambient atmosphere and thus is unable to generate an overpressure preventing the gas to flow through it in both directions) . Due to the continuous supply of oxygen, the first part of the gas supplied to the patient during the initial part of inhalation will be highly enriched with additional gas and may even be comprised almost entirely of the additional gas.
Depending on the tidal volume inspired by the patient, the amount of the additional gas will be reduced during the remaining part of the inhalation. The reduction will depend on the flow of air from the container and the flow of the supplied additional gas. The respiratory gas flow is normally irregular during an inhalation and the concentration of the additional gas over the entire inhalation phase will thus vary to a high degree. The manner of breathing (few deep breaths or many shallow breaths) will determine the minute volume of the additional gas inhaled by the patient.
It is therefore desirable to be able to supply a gas having a homogeneous mixture of air and the additional gas and it is also desirable to be able to select a specific concentration of the additional gas.
The advanced ventilators referred to above usually have servo controlled valves which can respond quickly and accurately for providing exact mixtures of different gases. It would thus be possible to include an advanced servo controlled valve or similar and a flow meter between the container and the patient in the described known apparatus.
By measuring the flow of air to the patient the valve could be controlled to supply an amount of additional gas based on the measured flow, which would result in a desired concentration of additional gas in the air. Since the flow may vary significantly during an inspiration, however, and may momentarily reach flow values of several liters per second, the valve must be both very fast and very accurate in order to supply the required amount of additional gas. Although such valves do exist, they are usually very expensive, require a relatively complex regulation circuit and are also more power demanding than other valves. Further, they usually also require that the additional gas be taken from a high pressure source in order to operate satisfactorily.